The present inventions are related to detection of data in a communication system, and more particularly to detection of servo repeatable run out information from a channel.
A read channel integrated circuit is a component of a magnetic storage device. In operation, a read channel component converts and encodes data to enable read/write head assembly to write data to a disk and then read back the data accurately. In, for example, a hard disk drive, the disk typically includes many tracks containing encoded data that extend around the disk in a radial pattern. Each track includes one or more of user data sectors as well as intervening servo data sectors. The information of the servo data sectors is used to position the read/write head assembly in relation to the disks so that the information stored in the user data sectors may be retrieved accurately.
Repeatable run out (RRO) refers to a phenomenon that occurs due to imperfect spindles. Imperfect spindles might not allow the disk to spin in an exact circle around the disk's center. If the disk is not rotating at the center, the track rotating under the head does not follow a circular trajectory, and hence the head might not be able to read the servo information properly. A similar phenomenon occurs when spindle imperfections were present at the time the servo information was written to the disk. Even though the disk may spin properly in a different hard disk drive while reading the servo information, since the information was not written properly on a circular track, the head might not be able to read the servo information accurately. Thus, there is a need for a mechanism to properly guide the head to follow the trajectory of the track. A RRO data field in the servo information serves this purpose.
FIGS. 1a-1d illustrate one form of RRO (termed a one “f” run out) that results from an imperfect spindle. FIG. 1a shows radial position versus error when the error is zero. The error will be zero when the read/write head assembly is tracking in a circular trajectory. FIG. 1b shows the read/write head assembly tracking a circular trajectory shown by the dashed circle 102 on disk 103. As shown in FIG. 1c, the error for one “f” run out varies as a function of the radial position, but the error at a given position repeats after one revolution of the disk. As shown in FIG. 1d, the head may track an oval path shown by the dashed path 104 on disk 103. This oval path results in what can be referred to as a “wobble” in the error signal. Since the wobble “repeats” itself from one revolution to another, techniques may be devised to compensate for the problem. By feeding positioning information about the repeatable error to servo control circuitry, the error may be corrected to position the head properly over the servo track. State of the art magnetic recording systems employ digital signal processing to detect servo data as opposed to older systems employing analog techniques.
Various systems have been developed to compensate for wobble that rely on writing a data pattern in the user data area that allows for detecting wobble in one or more regions of user data spread out around the disk. Such an approach includes writing a preamble pattern that is asynchronous to the sector data and allows for synchronizing to and detection of a subsequent RRO field. The RRO field is used to compensate for wobble. FIG. 2 depicts a data set 200 including both a servo data region 240 and user data regions 230, 250. User data region 230 includes user data 202, and user data region 250 includes user data 218 preceded by asynchronous RRO data. The asynchronous RRO data includes, among other things, an asynchronous preamble 214 and RRO address mark 215 used for synchronizing to subsequent RRO data 216. Servo data region 240 includes a servo preamble 204, a servo address mark 206, a Gray code 208 and burst fields 210, 212. Various spacers (SP) may be interspersed in the fields to address timing concerns. Such an approach allows for wobble compensation, however, it involves appreciable overhead including an asynchronous preamble and an RRO address mark. Further, a spacer is often required between the end of the servo data region and the beginning of the asynchronous RRO data. This overhead reduces the usable bit density of a storage device.
Hence, for at least the aforementioned reasons, there exists a need in the art for advanced systems and methods for providing wobble compensation.